This invention relates to an air/fuel ratio control system for performing feedback control of the air/fuel ratio of a mixture being supplied to an internal combustion engine, and more particularly to a device provided in the above system for correcting the position of a pulse motor used as an actuator for driving an air/fuel ratio control valve, during open loop control as a function of atmospheric pressure.
A system has already been proposed e.g. by the assignee of the present application, which includes an O.sub.2 sensor for detecting the concentration of oxygen present in the exhaust gases emitted from an internal combustion engine, fuel quantity adjusting means including a carburetor and operable to produce the mixture being supplied to the engine, and means operatively connecting the O.sub.2 sensor with the fuel quantity adjusting means in a manner effecting feedback control operation in response to an output signal produced by the O.sub.2 sensor to control the air/fuel ratio of the mixture to a preset value. The connecting means comprises an electrical circuit, an air/fuel ratio control valve, a pulse motor for driving the control valve, etc.
According to the above conventional air/fuel ratio control system, if the above feedback control operation is conducted in response to the output signal of the O.sub.2 sensor when the engine is under particular operating conditions (the start of the engine, wide-open-throttle, idle, deceleration, and acceleration at the standing start of the engine), often the air/fuel ratio is not controlled to a proper or desired value. Therefore, to achieve a proper or desired air/fuel ratio even under such a particular engine operating condition, it is necessary to relieve the air/fuel ratio control system of its closed loop mode where the air/fuel ratio control is carried out, and effect air/fuel ratio control in open loop mode upon the occurrence of the particular engine operating condition to drive the pulse motor to a predetermined preset position appropriate respectively for the particular engine operating condition.
During such open loop control, the air/fuel ratio of the mixture is kept at a predetermined constant value corresponding to the above predetermined preset position of the pulse motor. However, if there is a change in the ambient atmospheric pressure, the air/fuel ratio obtained does not have a proper value appropriate for the particular engine operating condition concerned, resulting in inferior engine performance.
In addition, the above predetermined preset position to which the pulse motor is set during open loop control under the particular engine condition form an initial pulse motor position with which closed loop control is initiated immediately following the open loop control. Therefore, to obtain optimum exhaust gas emission characteristics during the above open loop control and during transition from the open loop control to the closed loop control irrespective of changes in the ambient atmospheric pressure, the pulse motor position has to be compensated for changes in the ambient atmospheric pressure during open loop control.